It is well-known that a direct positive photographic image can be formed by using a silver halide light sensitive photographic material without any intermediate processing step of treatment or development of any negative photographic image.
The known processes which have been applied to form a positive image by using a positive type silver halide light-sensitive photographic material, excluding special processes, can be mainly classified into the two types.
The first type of process employs a fogged silver halide emulsion and decays the fog specks (latent image) at an exposed area by taking advantage of the solarization or Herschel effect, etc., to form a positive image after development.
Another type of process employs a silver halide emulsion free of fog and carries out a surface development while or after conducting fogging treatment to form a positive image. The fogging treatment may be carried out by conducting whole-surface exposure; using a fogging agent; using an oxidized developer or a developer containing hydrazine or a hydrazine derivative; etc. Usually, the silver halide emulsions employed in the second type of process are of the internal latent image type. After imagewise exposure, the silver halide grains are developed with a surface developer, i.e. one which leaves the latent image sites within the silver halide grains substantially undeveloped. Simultaneously, either by uniform exposure or, preferably, by the use of a nucleating agent, the silver halide grains are subjected to development conditions that would cause fogging of a negative-working photographic element. The internal latent image-forming silver halide grains which received actinic radiations during imagewise exposure develop under such conditions at a comparatively slow rate, as compared to the internal latent image-forming silver halide grains non-imagewise exposed. The result is a direct positive silver image.
In general the latter type of process shows higher sensitivity as compared to the former. However, the latter suffers from poor stability and, when employed in multilayer color light-sensitive materials, variations in maximum density and interference with spectral sensitization, as described for example in U.S. Pat. No. 4,115,122.